1. Field of the Invention
This invention is in the field of the prevention and treatment of diabetes mellitus.
2. Description of the Prior Art
Diabetes mellitus (DM) is a major chronic illness found in humans with many consequences. Some complications arising from long-standing diabetes are blindness, kidney failure, and limb amputations. Insulin-dependent diabetes mellitus (IDDM) accounts for 10 to 15% of all cases of diabetes mellitus. The action of IDDM is to cause hyperglycemia (elevated blood glucose concentration) and a tendency towards diabetic ketoacidosis (DKA). Currently treatment requires chronic administration of insulin. Non-insulin dependent diabetes mellitus (NIDDM) is marked by hyperglycemia that is not linked with DKA. Sporadic or persistent incidence of hyperglycemia can be controlled by administering insulin. Uncontrolled hyperglycemia can damage the cells of the pancreas which produce insulin (the .beta.-islet cells) and in the long term create greater insulin deficiencies. Currently, oral sulfonylureas and insulin are the only two therapeutic agents available in the United States. for treatment of Diabetes mellitus. Both agents have the potential for producing hypoglycemia as a side effect, reducing the blood glucose concentration to dangerous levels. There is no generally applicable and consistently effective means of maintaining an essentially normal fluctuation in glucose levels in DM. The resultant treatment attempts to minimize the risks of hypoglycemia while keeping the glucose levels below a target value. The drug regimen is combined with control of dietary intake of carbohydrates to keep glucose levels in control.
A fragment of human peptide molecule called, glucagon-like peptide-1 (GLP-1) has been found to be a glucose-dependent insulinotropic agent (Gutniak, M., et al. N. Engl. J. Bled. 1992; 326:1316-1322). GLP-1 is itself a fragment of the human proglucagon molecule. Another active fragment, glucagon-like insulinotropic peptide (GLIP), corresponds to GLP-1(7-36). It was reasoned that since GLIP is the naturally active form found in humans after a meal, this peptide may aid in glucose regulation in IDDM and NIDDM.
In normal subjects, the infusion of GLIP significantly lowered the meal-related increases in blood glucose concentration, and the plasma concentrations of insulin and glucagon. In patients with NIDDM, the treatment reduced the requirement for insulin by 8 fold. In patients with IDDM, the GLIP treatment lowered the insulin required by one half. This glucose-dependent activity is a very desirable characteristic for a therapeutic agent that can be used to treat DM avoiding tile complications of hypoglycemic side effects.
In 1981, it was discovered that Gila monster (Heloderma suspectum) venom stimulated pancreatic enzyme secretion in vitro (Raufman, J. P., et al., Gastroenterology 80:1257 abst. (1981); Raufman, J. P., et al., Am. J. Physiol. 242: G470-G474 (1982)). Several peptides have been isolated from the venom that can stimulate increased cAMP and amylase release from dispersed pancreatic acinar cells. These structural analogs to the mammalian peptides VIP (vasoactive intestinal peptide) and secretin include helospectin-I, helospectin-II (Parker, D. S. et al., J. Biol. Chem. 259:11751-11755 (1984)),and helodermin (Hoshino, M. et al., FEBS Lett. 178:233-239 (1984)). Recently, we discovered another peptide that increases cAMP and stimulated the release of amylase in dispersed acinar cells. This peptide was found in Heloderma horridum venom and was termed exendin-3 (Eng, J. et al., J. BIol. Chem. 265: 20259-20262 (1990). Exendin-3 shares homology with VIP, secretin, helospectin-I and -II, and helodermin. The venom of Heloderma suspectum was examined and another peptide was purified from it. This peptide called exendin-4 is an analogue of exendin-3 with an identical sequence except for substitutions in residues 2 and 3 from the amino terminus (Eng, J. et al.,J. Biol. Chem. 267:742-7405 (1992)). Experiments were done to establish that the exendins could stimulate cAMP activity in dispersed pancreatic acinar cells, and a specific antagonist, exendin(9-39) amide, which can inhibit the effects of the exendins, was identified. (Raufman, J. P. et al., J. BIol. Chem. 266: 2897-2902 (1991 )) Experiments were performed to establish that GLP-1 could interact with possible exendin receptors in dispersed pancreatic acinar cells in vitro (Raufman, J. P. et al., J. BIol. Chem. 267:21432-21437 (1992)).